Relatively inexpensive plenum or plug-type fans are well known in the industrial and commercial fan industry. They are commonly sold as an unhoused fan unit by the manufacturers although they are mounted in a suitable support structure that can include a front wall with an air inlet opening formed therein. These fans are used instead of, or to replace, centrifugal type fans which are commonly used in the air handling industry. The wheel of the plenum fan is used to pressurize a surrounding air plenum or housing in which the fan is installed. A number of air ducts can be connected to the housing and these can extend from any direction. In addition to being a reasonably inexpensive fan structure, a plenum or plug fan unit can save space by eliminating a special fan housing, transitions and diffusers commonly characterizing centrifugal air handling system. When required, two or more of these fans can be mounted side-by-side on common or separate support frames.
A common and well known difficulty of plug or plenum fans is that they can be inefficient in their operation and noisy compared to other types of fans. Furthermore, such assemblies are known to require considerably more electrical power for operation of the one or more fans than more efficient units that produce the same amount of or more air flow. With respect to the noise problem, it is noted that with many known plug type fans, low frequency noises are generally produced, and there is no currently available and practical solution to the noise problem. Traditionally, noise reduction in air moving assemblies has been achieved at the cost of dynamic performance, via the inclusion of dissipative silencers. Such silencers typically comprise acoustically lined fan housings, ducting, etc. In an air handling system, such structures create a static pressure drop which results in a lowered static efficiency. Furthermore, dissipative silencers are ill suited to reduce or eliminate the tone component of sound, namely, blade pass frequency tone.
U.S. Pat. No. 5,749,702 (Datta et al.) describes, among other things, a fixed center body for axially directing air flow to and within a bladed fan wheel having an annular air outlet. The center body extends through the axial air intake, and radially expands rearwardly, terminating at an end at the back of the fan wheel, close to the rear plate thereof. Both inlet and outlet structures, including the center body, incorporate sound attenuating material for broadband noise reduction. Increased fan wheel efficiency is alleged as attributable to a solid curved rear end section of the center body, which redirects air flow in a radial direction towards the annular outlet of the fan wheel. Furthermore, a wall, spaced from a fixed sidewall or front wall of a fan support structure so as to be positioned behind the fan wheel, is further provided. The additional wall is preferably filled with sound attenuating material, and more preferably still, has a perforated front surface facing the back plate of the fan wheel.
U.S. Pat. No. 5,426,268 (Yazici et al.) describes combined utilization of air duct inlet and outlet silencer apparatuses for an air handling system. Both apparatuses include interior walls, arranged between interior and exterior walls thereof, comprised of sound attenuating material, with at least portions of the interior walls constructed of perforated metal sheets. In the outlet duct apparatus, the main passageway is substantially straight and increases in transverse cross-section from the inlet to the outlet. The transverse cross-section changes from circular at the end of the passageway adjacent the fan to rectangular at the opposite end.
U.S. Pat. No. 5,066,194 (Amr et al.) describes a fan orifice structure intended for use in conjunction with an outside enclosure, usually containing a heat exchanger and compressor of an air conditioner. The orifice is defined by an annular curved surface that extends downwardly from a top wall of the cover. The curved surface is generated by rotating a planar and curvilinear line about a coplanar axis of generation. It is said that the contour of the orifice enhances fan efficiency and reduces radiated noise. The orifice cover is made from plastic materials by a molding process.
U.S. Pat. No. 4,576,549 (Lanier) is generally directed to a centrifugal fan having a plurality of vortex generators fixed onto the outer wall of an annular member leading into an air inlet of the fan wheel. An inlet cone is shown as a concave annular form tapered inwardly from the larger diameter air inlet in the fan wheel plate. Vortex generators are shown as formed plates having lateral edges contoured to fit the curved annular wall of the inlet cone. It is believed that such structures, so arranged, permit merger of skin friction induced air current with the lower velocity air being discharged from the rotating fan wheel blades
As is readily appreciated, it remains advantageous to provide a fan unit which is simple to build and construct which employs a bladed fan wheel having an axial air intake and an annular air outlet, and at least one outlet diffuser for directing airflow from the fan wheel such that static efficiency is improved, and noise is greatly reduced. It is further advantageous to enhance the noise reduction capabilities of fan assemblies for air handling systems, more particularly, both the broadband and tone aspects thereof.